Studies are concerned with the mechanisms which control the hepatic response to massive resection. A tachnique has been employed for monitoring this response by percutaneous liver biopsy during regeneration. Cores of fresh liver tissue are incubated in precursor-labeling media and autoradiographic or quantitative specific activity studies carried out. Variations in uptake of label in such biopsy cores indicate the stage in regeneration, extent of hepatic resection and other variants in the regenerative response. At the present time attempts to "accelerate" or condense the regenerative response through the use of prior "stress" and drugs are under investigation. The unusual in vitro growth properties of cells removed from active regenerating liver continues to be a subject of study. These cells have been found to form colonies in soft agar culture and also produce spleen colonies in lethally radiated mice. Cells from regenerating liver must be removed approximately 48 hours post resection in the rat and 5 to 7 days post resection in the mouse for these studies to be effective. Isolation perfusion of the oxygenated rat liver with precursor labeling suspensions which simulate blood (and include RBC) results in a 100 fold increase in uptake in regenerating (24 hrs. 68% resected rat) as opposed to normal or sham operated livers. This system will be employed to evaluate factors which may be expected to accelerate the hepatic response to resection. BIBLIOGRAPHIC REFERENCES: Hays, D.M., Sera, Y., Koga, Y., and Hays, E.F.: Formation of Colonies in Soft Agar Medium by Regenerating Liver Cells, Proc. Soc. Exper. Biol. and Med. 148:596-599, 1975. Hays, E.F., Firkin, F.C., Koga, Y. and Hays, D.M.: Hemopoietic Colony Forming Cells in Regenerating Mouse Liver. J. Cell. Phy. 86:213-220, 1975.